Coronary heart diseases are one of the main causes of death in the industrialized world. They are often triggered by atherosclerotic plaque which gathers in the coronary vessels and which can lead to narrowing or occlusion of the vessels. Atherosclerotic plaque can be divided into various types with different compositions.
Lipid-rich or noncalcified plaque, also referred to as soft plaque, is associated with a particularly high risk of a coronary event such as an infarct or sudden cardiac death, because its rupture most likely leads to an acute vascular occlusion. In cases where soft plaque is present, the risk of an acute coronary event can be reduced by using certain medicines called lipid-lowering agents. In contrast to soft plaque, another type of plaque called calcified plaque more rarely causes acute vascular occlusions. The same applies to fibrous plaque, an intermediate stage between soft plaque and calcified plaque.
When using imaging techniques, it is therefore of advantage to be able to detect the presence of soft plaque in the patient's coronary vessels as quickly as possible. Known imaging methods for visualizing soft plaque in coronary vessels are the invasive imaging methods of intravascular ultrasound imaging (IVUS) or optical coherence tomography (OCT). These imaging techniques generate gray-scale images whose image plane is oriented perpendicular to the vessel axis. The vessel can be seen as a concentric ring in the center of the image, and different plaque types can be pinpointed by different gray-scale scale areas in the image. However, the observer must have considerable experience to reliably detect the presence of plaque and to be able to differentiate between the different types of plaque.
Since the introduction of multi-slice computed tomography machines, which can record four or more slices simultaneously by way of a suitable detector array, noninvasive imaging of the heart is also possible in conjunction with electrocardiographically synchronized operation (ECG gating). ECG gating, in conjunction with the high recording speed of a multi-slice computed tomography machine, permits visualization of the coronary arteries with minimal movement artifacts. The recorded 2D slice images can then be visualized in different ways, for example by 3D volume rendering (VRT) or by two-dimensional thin-slice MIP (maximal intensity projection).
Examples of tomographic imaging of this kind can be found in the publication by B. Ohnesorge et al., “Cardiac Imaging by Means of Electrocardiographically Gated Multisection Spiral CT: Initial Experience”, Radiology (2002), volume 217, pages 564-571. As is set forth in the publication by C. R. Becker et al., “Current Development of Cardiac Imaging with Multidetector-Row CT”, European Journal of Radiology (2000), volume 36, pages 97-103, calcified and noncalcified coronary plaque can also be presented using multi-slice computed tomography on the basis of approximately isotropic slice image data sets. From S. Schröder et al., “Non-invasive Detection and Evaluation of Atherosclerotic Plaque with Multi-Slice Computed Tomography”, Journal of the American College of Cardiology (2001), volume 37, pages 1430-1435, a method for visualizing deposits in coronary vessels is known in which lipid-rich, fibrous and calcified plaque can be differentiated by determining the CT density values in the image data of the reconstructed 2D slice images. In this method, density measurements are carried out at 16 randomly selected points in at least 4 different axial slice images of each area of plaque.
However, when viewing the 2D slice images of the examined area which have been obtained with the imaging tomographic technique, a problem which often arises is that of the poor level of detection of the different types of plaque in relation to the surrounding tissue.